Field of the Invention
The present invention relates to an apparatus and method for measuring radiation, and relates particularly, but not exclusively, to an apparatus and method for scanning a structure to detect changes in density between different parts of the structure. The apparatus and method of the invention have particular benefits for use in an undersea pipeline inspection apparatus.
Description of the Related Art
It is known to use gamma radiation for scanning structures, for example to obtain information about the density within the structure or to identify flaws such as cracks or corrosion in a structure. This is particularly useful for inspecting pipes subsea, where it is not always possible to inspect the pipe from the interior. Gamma scanning is also used for obtaining information about other industrial structures such as distillation columns and the like.
An apparatus for scanning structures such as a pipeline or process vessel using gamma radiation is described in GB 2496736 A. This apparatus comprises a source of gamma radiation and an array of detectors. The apparatus is capable of being arranged with the structure to be scanned, such as a pipeline, positioned between the source and detectors so that radiation emitted by the source can pass along a plurality of paths through a portion of the structure to the detectors. The number of detectors in the array may range from fewer than 10 up to more than 100, e.g. up to 150, depending on the application. To obtain high resolution data, a large number of detectors are used, closely spaced from one another. The detectors are arranged in the form of an arc centred on the structure to be scanned. In operation, the source and array of detectors are arranged in fixed relationship with respect to each other, and are rotated around the structure to be scanned. In this way, information about the density of the structure along a plurality of paths is obtained, enabling a high resolution density map of the structure to be calculated. This technique has similarities with medical imaging techniques such as x-ray tomography.
When deploying this gamma scanning techniques in a subsea environment, there are additional challenges which do not arise with land-based measurements. When operating subsea at a depth of 1000 metres the pressure is 100 atmospheres. For each additional 1000 metres of depth below sea level, the pressure increases by a further 100 atmospheres. The apparatus must be able to withstand this pressure. Furthermore the apparatus must be compact for deployment subsea using submarines capable of operating at the required depth. It is challenging to package all the required components of the apparatus into a sufficiently small volume. In order to deploy this technique at even greater depths, it is necessary to meet ever more stringent requirements, particularly regarding the size of the apparatus.
A typical detector for detecting gamma radiation comprises a scintillating crystal and a photodetector. Gamma rays entering the scintillation crystal interact with the scintillating material to produce photons in the visible and/or ultra violet region. These scintillation photons are detected using a photodetector, typically a photomultiplier tube (PMT), which outputs an electrical pulse. The electrical pulse provides information about the number and energy of the incident gamma photons. Counting the number of electrical pulses corresponding to gamma rays transmitted from the source to the detector, through the structure being scanned, enables differences in the density of different parts of the structure to be detected.
Integrating a large number of photodetectors into an apparatus for use subsea is one of the many challenges of designing such an apparatus. Photomultiplier tubes are preferred due to their high sensitivity to low light levels. However, photomultiplier tubes comprise vacuum tubes which must be sealed against the high pressure encountered at depth. Photomultiplier tubes also require high voltages (˜1 kV) for biasing the dynodes of the photomultiplier tube and these high voltages must be effectively isolated for subsea operation. The power supply for each photomultiplier tube must also be very stable because the gain or calibration of photomultiplier tubes is very sensitive to changes in the high voltage biasing voltage.